Ronaldo blamed the high altitude after Ecuador ended Brazil's 10-match unbeaten run in World Cup qualifiers on Wednesday.
"I had a big headache, I was short of air and I needed oxygen to breathe," the Brazil striker told reporters following Ecuador's 1-0 win.
Ronaldo, despite his suffering, played the full 90 minutes and wasted an excellent late chance to equalise for the world champions.
"I'm here, giving everything for the national team," added the Real Madrid player, who was due to fly straight back to Spain for his club's title clash with Barcelona on Saturday.
Brazil's defeat, which took place 2,800 metres above sea level, means they could also lose first place in the 10-nation South American qualifying group if Argentina win at home to Venezuela later on Wednesday.
But they remain comfortably on course for a top-four finish and a berth in the 2006 finals in Germany.
"The important thing is to qualify," said coach Carlos Alberto Parreira. "Losing first place and losing the leadership doesn't mean anything.
HIGH-RISK GAMES
"We knew this was one of our high-risk games. They improved in the second half. We lost the game, we didn't want to, but it happened."
Ecuador coach Luis Fernando Suarez was delighted with the result.
"It's good, from any point of view, to beat the world champions," he said. "This team has got a lot of good things, a lot of organisation.
"It was very tense but, when we made mistakes, Brazil didn't take advantage."
Striker Ivan Kaviedes, who scored the only goal when Ecuador beat Brazil for the first time three years ago, said the victory was a reflection of his side's recent improvement.
"It's no coincidence that we beat Brazil," he said. "We're still alive and we want the people to enjoy it."
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